docs: def _compute_soil_amplification(self, C, vs30, pga_bc, mean):
          """
          Compute soil amplification, that is S term in equation (5), p. 2191,
          and add to mean values for non hard rock sites.
          """
          # convert from base e (as defined in BA2008) to base 10 (as used in
          # AB2006)
          sal = np.log10(np.exp(self._get_site_amplification_linear(vs30, C)))
          sanl = np.log10(np.exp(
              self._get_site_amplification_non_linear(vs30, pga_bc, C)))
  
          idxs = vs30 < 2000.0
          mean[idxs] = mean[idxs] + sal[idxs] + sanl[idxs]